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POLYFLOW - Pressure build up across a section of twin screw extruder

For the analysis of flow around screws with the use of the MST algorithm, it is often recommended to select fn=fs=0 as boundary conditions at the inlet and outlet. Does this mean that pressure will be zero at the inlet and the outlet? If so, does it mean that these boundary conditions cannot predict pressure build up across the section?
If you impose zero forces at inlet and at outlet, for a given screw angular velocity, you indeed get a zero pressure build-up (by definition), but the relevance of such a simulation is the evaluation of the flow rate for such situation.
If you want to observe a pressure buildup, the flow simulation should be changed as follows:
a) for a given screw angular velocity (constant), perform a simulation with an evolution on the inlet flow rate.
b) impose a zero normal force at the outlet.
After completion of the calculation, you can build a graph showing the pressure buildup vs. flow rate.
Note that vanishing forces are preferred in the tangential direction, for avoiding possible incompatibilities.